How do you draw in multiple colors with grease pencil?
First, make sure that you are in material preview shading or rendered shading (unfortunately you cut the top of the viewport on the image on the question, don't do that).

Color will not appear in solid view.


For Material.
Set the material for the stroke and fill in the material properties window.


For vertex color
Set the color directly either on the top color selector on the screen.

Or use the tool options in the properties window.